Testing visitors to Britain for coronavirus would not circumvent the fresh requirement for them to quarantine for 14 days after their arrival, the chief scientific adviser has suggested.

Home Secretary Priti Patel announced on Friday that self-isolation periods of two weeks will be imposed on new arrivals to the UK from June 8, with fines for anyone who breaches the measure designed to prevent new waves of coronavirus from overseas.

Speaking at the daily Downing Street briefing on Covid-19, Sir Patrick Vallance, the UK Government’s chief scientific adviser, said test results were not conclusive enough to catch every case of coronavirus.

He explained that the swab tests being used were not able to decipher whether someone was in the early stages of having contracted coronavirus, a period when those carrying the virus are likely to be asymptomatic.

Testing somebody and saying, ‘You’re negative’, doesn’t tell you if somebody is about to get it in a week’s timeSir Patrick Vallance

“The reason a negative test is not very predictive is if you’ve just caught the disease, you incubate it for a few days when you will test negative,” he said.

“You start to test positive maybe at around five days, maybe a bit sooner, and you may be shedding a lot of virus for a couple of days then and for a few days afterwards.

“And then you gradually recover and you may not shed.

“So, clearly, it depends on the time at which you caught the infection as to when you expect a positive test.

“That means just testing somebody and saying, ‘You’re negative’, doesn’t tell you if somebody is about to get it in a week’s time.”

Advice published by the Home Office said that “for the moment, testing at the border is not a part of” the protection measures.

Answering why Britain was not introducing temperature screening at the border like other countries had, the Home Office added: “Temperature testing will not identify everyone who is infected with coronavirus when they enter the UK – especially people who have not yet developed symptoms or for who raised temperature is not a symptom.

“That is why we will be asking all travellers to self-isolate for 14 days.”

Paul Lincoln, director general of the Border Force, warned prospective travellers that they could be putting lives at risk if they decide to travel with symptoms associated with the deadly disease.

“The advice is quite clear,” he said at the press briefing.

“If you have the virus or are displaying the symptoms, or if you’ve been in contact with someone with the virus, you should not travel.

“To do so otherwise is potentially putting people’s lives at risk.”

Ms Patel reiterated that Government advice continues to be that all but essential foreign travel should be avoided.